1. Field of the Invention
The present invention relates to a slidable ceramic member having a sliding surface covered with a solid lubricant and a method of manufacturing such a slidable ceramic member.
2. Description of the Prior Art
It is known that the sliding characteristics of a sliding surface can be improved by covering the sliding surface with a solid lubricant such as calcium fluoride (CaF.sub.2) or barium fluoride (BaF.sub.2).
Such a solid lubricant may be coated by the well known plasma spray coating process. The solid lubricant may also be coated by another coating process as disclosed in page 143 and following pages of Solid Lubrication Handbook published by Saiwai Shobou.
According to the disclosed coating process, a slidable member made of a nickel alloy is heated to oxidize the sliding surface, thus forming an oxide layer thereon. Then, a kneaded mixture of an inorganic binder of CoO--B.sub.2 O.sub.3 -BaO and calcium fluoride (CaF.sub.2) is sprayed on the sliding surface which is cooled to 150.degree. C. Thereafter, the sliding surface is heated again to 1090.degree. C. to fire the coated kneaded mixture on the sliding surface.
If a sliding surface of a ceramic member made of silicon nitride (Si.sub.3 N.sub.4) or silicon carbide (SiC) is coated with calcium fluoride (CaF.sub.2) by the plasma spray process, then the coated layer of calcium fluoride (CaF.sub.2) may subsequently be peeled off the sliding surface due to a temperature change after the spray coating process because of the different coefficients of thermal expansion of silicon nitride (Si.sub.3 N.sub.4) or silicon carbide (SiC) and calcium fluoride (CaF.sub.2).
The coating process disclosed in the aforesaid literature is problematic in that since an inorganic binder is used, the proportion of calcium fluoride (CaF.sub.2) in the coated surface layer is small, and hence the sliding characteristics of the coated surface layer are poorer than a coated surface layer made of only calcium fluoride (CaF.sub.2).